1. Field of the Invention
The present invention relates to novel classes of proteins and protein analogues which bind to and may inhibit kallikrein.
2. Description of the Background Art
Kallikreins are serine proteases found in both tissues and plasma. Plasma kallikrein is involved in contact-activated (intrinsic pathway) coagulation, fibrinolysis, hypotension, and inflammation. (See Bhoola, et al. (BHOO92)). These effects of kallikrein are mediated through the activities of three distinct physiological substrates:
i) Factor XII (coagulation), PA1 ii) Pro-urokinase/plasminogen (fibrinolysis), and PA1 iii) Kininogens (hypotension and inflammation).
Kallikrein cleavage of kininogens results in the production of kinins, small highly potent bioactive peptides. The kinins act through cell surface receptors, designated BK-1 and BK-2, present on a variety of cell types including endothelia, epithelia, smooth muscle, neural, glandular and hematopoietic. Intracellular heterotrimeric G-proteins link the kinin receptors to second messenger pathways including nitric oxide, adenyl cyclase, phospholipase A.sub.2 and phospholipase C. Among the significant physiological activities of kinins are: (i) increased vascular permeability; (ii) vasodilation; (iii) bronchospasm; and (iv) pain induction. Thus, kinins mediate the life-threatening vascular shock and edema associated with bacteremia (sepsis) or trauma, the edema and airway hyperreactivity of asthma, and both inflammatory and neurogenic pain associated with tissue injury. The consequences of inappropriate plasma kallikrein activity and resultant kinin production are dramatically illustrated in patients with hereditary angioedema (HA). HA is due to a genetic deficiency of C1-inhibitor, the principal endogenous inhibitor of plasma kallikrein. Symptoms of HA include edema of the skin, subcutaneous tissues and gastrointestinal tract, and abdominal pain and vomiting. Nearly one-third of HA patients die by suffocation due to edema of the larynx and upper respiratory tract. Kallikrein is secreted as a zymogen (prekallikrein) that circulates as an inactive molecule until activated by a proteolytic event, Genebank entry P03952 shows Human Plasma Prekallikrein.
Mature plasma Kallikrein contains 619 amino acids. Hydrolysis of a single Arg-Ile bond (at positions 371-372) results in the formation of a two-chain proteinase molecule held together by a disulfide bond. The heavy chain (371 amino acids) comprises four domains arranged in sequential tandems of 90-91 residues. Each of the four domains is bridged by 6 half-cysteine residues, except the last one, which carries two additional half-cysteine residues to link together the heavy and light chains.
These domains are similar in sequence to factor XI. The light chain (248 residues) carries the catalytic site, and the catalytic triad of His-415, Asp-464 and Ser-559 is especially noteworthy.
The most important inhibitor of plasma kallikrein (pKA) in vivo is the C1 inhibitor; see SCHM87, pp.27-28. C1 is a serpin and forms an irreversible or nearly irreversible complex with pKA. Although bovine pancreatic trypsin inhibitor (BPTI) (SEQ ID NO:1) was first said to be a strong pKA inhibitor with K.sub.i =320 pM (AUER88), a more recent report (Berndt, et al., Biochemistry, 32:4564-70, 1993) indicates that its Ki for plasma Kallikrein is 30 nM (i.e., 30,000 pM). The G36S mutant had a Ki of over 500 nM.
"Protein engineering" is the art of manipulating the sequence of a protein in order to alter its binding characteristics. The factors affecting protein binding are known, but designing new complementary surfaces has proved difficult. Although some rules have been developed for substituting side groups, the side groups of proteins are floppy and it is difficult to predict what conformation a new side group will take. Further, the forces that bind proteins to other molecules are all relatively weak and it is difficult to predict the effects of these forces.
Nonetheless, there have been some isolated successes. Wilkinson et al. reported that a mutant of the tyrosyl tRNA synthetase of Bacillus stearothermophilus with the mutation Thr.sub.51.fwdarw.Pro exhibits a 100-fold increase in affinity for ATP. Tan and Kaiser and Tschesche et al. showed that changing a single amino acid in a protein greatly reduces its binding to trypsin, but that some of the mutants retained the parental characteristic of binding to an inhibiting chymotrypsin, while others exhibited new binding to elastase.
Early techniques of mutating proteins involved manipulations at the amino acid sequence level. In the semisynthetic method, the protein was cleaved into two fragments, a residue removed from the new end of one fragment, the substitute residue added on in its place, and the modified fragment joined with the other, original fragment. Alternatively, the mutant protein could be synthesized in its entirety.
With the development of recombinant DNA techniques, it became possible to obtain a mutant protein by mutating the gene encoding the native protein and then expressing the mutated gene. Several mutagenesis strategies are known. One, "protein surgery", involves the introduction of one or more predetermined mutations within the gene of choice. A single polypeptide of completely predetermined sequence is expressed, and its binding characteristics are evaluated.
At the other extreme is random mutagenesis by means of relatively nonspecific mutagens such as radiation and various chemical agents, see Lehtovaara, E. P. Appln. 285,123, or by expression of highly degenerate DNA. It is also possible to follow an intermediate strategy in which some residues are kept constant, others are randomly mutated, and still others are mutated in a predetermined manner. This is called "variegation". See Ladner, et al. U.S. Pat. No. 5,220,409.
The use of site-specific mutagenesis, whether nonrandom or random, to obtain mutant binding proteins of improved activity, is known in the art, but does not guarantee that the mutant proteins will have the desired target specificity or affinity. Given the poor anti-kallikrein activity of BPTI, mutation of BPTI or other Kunitz domain proteins would not have been considered, prior to the present invention, a preferred method of obtaining a strong binder, let alone inhibitor, of kallikrein.